Organic Light Emitting Diode (OLED) display has many advantages including wider view angles, greater brightness, higher contrast, lower power consumption, thinner physical thickness over many conventional display technologies. Low Temperature Poly Silicon (LTPS) substrate with its fast electron mobility characteristics has become a popular substrate for making thin-film-transistors-based pixel driving circuit for driving light emission of each OLED associated with each sub-pixel in the display panel. In real OLED display apparatus, every thin-film transistor formed in the display panel may not have uniform characteristics in threshold voltage, carrier mobility, or resistor series, leading to non-uniform variations in image display across the display panel.